This invention relates to single sideband radio receivers and particularly to such receivers which are useful in microwave mobile radiotelephone systems.
There are a number of ways known in the art for extracting information signals from single sideband radio signals. One way involves the use of a so-called direct conversion operation. This is shown, for example, in the paper "Direct Conversion SSB Receivers: A Comparison of Possible Circuit Configurations for Speech Communication" by S. R. Al-Araji et al. in the Radio And Electronic Engineer, Vol. 43, No. 3, March, 1973, pages 209 through 215. An early form of a similar circuit that is useful for either a transmitter or a receiver is shown in the D. K. Weaver Jr., U.S. Pat. No. 2,928,055. Neither Al-Araji et al. nor Weaver deals with phase or gain correction considerations.
Rapid amplitude changes are characteristic of microwave mobile radiotelephone signals working in a Rayleigh fading environment. (The microwave radio range is usually considered to be the range between approximately 450 megahertz and 20 gigahertz.) Rapid fading results from the motion of a vehicle, carrying a radio used in a communication link, through a multipath radio field. At any one point in this field, radio signals arrive by many paths and from a wide range of directions. Because of the different time delays associated with the different paths, the phases of the incoming signals appear randomly distributed over 360 degrees. When those signals add up at the radio antenna, they may add either destructively, as in the case of a fade, or constructively. As the vehicle moves, the signal phases associated with the signals traveling the different paths to the antenna change; and the sum can fluctuate dramatically. Thus, the net received signal strength tends to drop markedly in strength at approximately half wavelength intervals. Rayleigh statistics often closely approximate the degree of fading on the signal. That is, the signal envelope spends about 10 percent of the time more than 10 dB below the average envelope power level and about 1 percent of the time more than 20 dB below the average envelope power level.
Such fast amplitude changes occur too rapidly to be corrected by conventional feedback automatic gain control (AGC) systems so as to provide telephone quality audio output signals. This is because propagation delays through receiver circuits to the point at which an AGC control signal is derived are of a size which is comparable to the fade recurrence interval size so that such rapid changes cannot be corrected by feedback techniques. An even stronger expression of the dismal prospects for AGC in regard to fast deep fading in microwave mobile radiotelephone amplitude modulation and single sideband (SSB) systems is to be found in the book Microwave Mobile Communications, edited by W. C. Jakes, Jr., John Wiley & Sons, New York, 1974, at page 207. Nevertheless, attempts have been made, usually at frequencies below the microwave range, to effect some measure of gain control using feedback techniques. Two examples are the "Potential of SSB For Land Mobile Radio" by R. W. Gibson et al., pages 90 through 94, of The 29th IEEE Vehicular Technology Conference Record, March 1979, and "AGC, AFC, Tone Select Circuits for Narrow-Band Mobile Radio", by B. B. Lusignan, the latter paper having been orally presented at the February 1979 International Telecommunications Exposition (Intelcom 1979) Dallas, Texas (copies hand distributed).
In a somewhat different approach, a feed forward technique for AGC has been used; and one example is found in "An Experimental Fast Acting AGC Circuit" by A. L. Hopper, at pages 13 through 20, of the IRE International Convention Record (USA), 10, part 8, May 1962. Two other examples are "Forward Feeding AGC With Extended Signal Delays", by A. J. Rawling et al., at pages 85 through 92, of the Institute of Electronic and Radio Engineers Conference Proceedings, No. 40, July 1978, and "Receivers for the Wolfson SSB/VHF Land Mobile Radio System" by W. Gosling et al. at pages 169-178 of the same Conference Proceedings. In all of these cases, however, the correction reference signal is derived in a way that does not cleanly separate the amplitude reference. Thus, none of these gain control techniques produces a signal which is of telephone quality in the microwave mobile radio region where problems of deep rapid fade are particularly severe.
The same multipath environment that produces the rapid fading also causes rapid extraneous phase fluctuations to be introduced in the received signal. The rapid phase and amplitude fluctuations introduced due to vehicle motion have the effect of spreading a single frequency tone into a band of frequencies with a bandwidth equal to twice the maximum Doppler frequency based on the vehicle speed. For example, a 160 hertz bandwidth is applicable for a vehicle speed of 60 miles per hour and a radio frequency of approximately 900 megahertz. The random phase or frequency variations are often referred to as "random FM." Alternatively, one can think of a number of signal components traveling different paths and coming in at different angles to the vehicle's direction of motion and, therefore, having different Doppler shifts. These signals then add up to occupy a band of frequencies. A further treatment of fading in the multipath environment can be found in "Advanced Mobile Phone Service: Voice and Data Transmission" by G. A. Arredondo et al. at pages 98-103 of the January 1979 Bell System Technical Journal. Regardless of the way of describing their cause, these phase fluctuations on the received signal alone are sufficient to produce substantial distortion in the recovered audio or other information signal which cannot be corrected by the usual feedback automatic frequency control (AFC) loops which are subject to circuit propagation delay effects as already mentioned in regard to AGC.
Some examples of systems for frequency control include the U.S. Pat. Nos. 3,275,940 of L. R. Kahn, 3,271,681 of R. J. McNair, and 3,634,766 of M. L. Boyer. In these patents, a received single sideband signal is translated down in frequency prior to separating a pilot component from the information component, and then the pilot signal is fed back to adjust a local oscillator frequency. Another example of a feedback frequency control system is found in the aforementioned Lusignan paper.
The prior art single sideband mobile radio experiments have generally worked at frequencies no higher than the lower edge of the microwave frequency range, i.e., the range between approximately 450 megahertz and 20 gigahertz. Consequently, they have not faced the more severe problems of rapid deep fading and fast Doppler related frequency variations that characterize the multipath environment of land mobile radio in the microwave frequency range.